How far are you trying to reduce your velocity? Knowing that might improve the quality of advice and suggestions that can be offered.
Two possible courses of action I can think of, depending on what velocity range you are after, are:
1. Go with a slower burning powder than is normally appropriate for the cartridge. Doing this will typically provide high load density and reduced velocity because the limits of case capacity are reached before the powder can generate full pressure.
2. If your velocity goals are low enough, use Trail Boss. It is a very bulky powder that supports reduced loads by its very nature. Most Trail Boss loads I have seen produced less than 2000 fps.